The invention relates to a device for controlling a motor comprising a rotor provided with a winding and a stator.
An apparatus wherein such a device is employed is described in European patent application EP 0 859 456 A1. This apparatus comprises an arm provided with a head for reading/writing information, which arm is intended to be made to move by the motor mentioned hereinabove, and a second, so-called disk motor intended to bring about rotation of a disk whose surface carries information, for example a magnetic hard disk for computers. Each motor is provided with input terminals, commonly referred to as supply terminals, which are connected to one or more windings intended to produce a magnetic field enabling the motor""s rotor to be set in motion. Each of the supply terminals is connected to a power supply circuit intended to supply energy to the winding during standard operation of the apparatus. When the power supply of the apparatus is suddenly interrupted, the rotor of the second motor, which is driven by the kinetic energy from the disk, keeps rotating, thereby generating a counter-electromotive force at its supply terminals, so that the second motor serves as a generator. This counter-electromotive force, which acts as a substitute for the supply voltage of the motor of the arm during a so-called emergency return phase, must enable said motor to move the arm towards a rest zone which is embodied such that the arm can rest there without causing damage to the read/write head.
In the known device, the value of the voltage applied to the winding of the motor of the arm is not controlled during said emergency return phase, which can be attributed to the fact that said voltage is intended to vary as a function of the rotational speed of the disk. This implies that the speed of the arm is not controlled either.
Consequently, there is a possibility that the arm is moved towards the rest zone at a high speed, which may cause the arm to collide with a stop intended to keep the arm in the rest position, which collision may cause the arm to rebound in the direction of the disk, causing the read head and the disk to violently contact each other, leading to possible damage of the two elements.
Conversely, if the speed of the arm is too low during the emergency return phase, the arm may not have sufficient time to complete its movement towards the rest zone before the counter-electromotive force generated by the second motor disappears following the end of the rotation of the disk, which occurs inevitably when its kinetic energy has dissipated entirely. In this case, the read/write head will land on the disk instead of on the rest zone, which may cause damage to the head as well as the disk.
For this reason, it is an object of the invention to overcome these drawbacks by providing a control device wherein the speed of the arm can be measured.
In accordance with the invention, a control device in accordance with the opening paragraph comprises:
control means which make it possible to manage the power supplied to the winding of the rotor of the motor of the arm, and
means for measuring the value of a counter-electromotive force induced into said winding by the movement of the rotor when the power supply of the winding of the rotor is interrupted.
In this control device, the counter-electromotive force generated by the rotor of the motor of the arm is measured for a brief period of time during which the winding is not energized. During this period of time, consequently, the speed of the arm is constant and proportional to the counter-electromotive force whose value is measured. The value measured by the measuring means thus is representative of the speed of the arm.
As the speed of the arm can be measured by virtue of the invention, it is possible to control said speed in order to make sure that the displacement of the arm towards the rest zone occurs in the best possible conditions.
For that purpose, the invention enables the speed of the arm to be measured at different instants during its travel towards the rest zone, to compare the values thus measured with predetermined values corresponding to an optimum travel, and to regulate the effective speed of the arm in order to make sure that the travel of the arm is in conformity with the optimum travel.
In such an embodiment of the invention, the control device described hereinabove additionally comprises comparison means intended to supply a command signal that is representative of a comparison between the value of a reference voltage and the value of the counter-electromotive force supplied by the measuring means, said control means being regulated by the command signal.
In a particular embodiment of the invention, the control means comprise a transistor whose main current path extends between a voltage source and a terminal of the winding of the rotor, the conduction of said transistor being controlled by the command signal.
This embodiment is advantageous because it provides a specific power supply circuit for the winding of the motor of the arm, which is activated during the emergency return phase and separated from the circuit responsible for supplying power to this winding during the standard mode of operation. As the specific power supply circuit is regulated by the command signal, the speed of the arm can be regulated readily and quasi instantly.
In accordance with a particularly advantageous embodiment of the invention, the value of the reference voltage varies monotonically in the course of time.
This embodiment enables the speed of the arm to be reduced in a continuous and controlled manner, such that said arm reaches the rest zone at a speed which is sufficiently low to avoid any rebound.
In accordance with the foregoing, the invention also relates, in one of its applications, to an apparatus enabling information to be written on the surface of a carrier provided for this purpose, and enabling said information to be read using a read/write head which is carried by an arm, which arm is intended to be made to move with respect to this carrier by means of a first motor, said carrier being intended to be made to move by means of a second motor, which apparatus is characterized in that the first motor is regulated by means of a control device as described hereinabove.